Routine echocardiographic examination of all patients suspected of having idiopathic hypertrophic subaortic stenosis (IHSS) has proven to be a valuable diagnostic tool in this clinic. Determination of the septal to free wall ratio and the detection of systolic anterior motion of the anterior mitral valve leaflet are the most essential portions of the examination. The purpose of this study is to critically evaluate sequential echocardiograms taken of patients undergoing left ventricular myotomy and myectomy. These data will be correlated with pre- and postoperative cardiac catheterization data and the subsequent clinical course of each patient. It is hoped that this study will demonstrate the following: first, that preoperative echocardiographic measurements of the left ventricular septum and free wall correlate closely with the findings at operation and/or necropsy; secondly, that the close proximity of the anterior mitral leaflet and septum during systole can be quantitated and correlated with the degree of outflow tract obstruction as determined by cardiac catheterization; and thirdly, that echocardiography is a useful tool in following and assessing the long-term results of myotomy and myectomy.